Method for limiting use of exceptional resource performed by user equipment in wireless communication system and user equipment using method

ABSTRACT

The present invention provides a method for the performance of a device-to-device (D2D) operation by a user equipment (UE) in a wireless communication system, the method comprising the steps of: starting to use an exceptional resource; determining whether a condition for suspending the use of the exceptional resource is satisfied; and when the condition for suspending the use of the exceptional resource is satisfied, suspending the use of the exceptional resource, wherein the condition for suspending the use of the exceptional resource is a suspension condition in an RRC idle state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/011241, filed on Oct. 22, 2015,which claims the benefit of U.S. Provisional Application Nos.62/066,907, filed on Oct. 22, 2014, and 62/076,920, filed on Nov. 7,2014, the contents of which are all hereby incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly relates to a D2D operation method performed by a userequipment (UE) in a wireless communication system and the UE using themethod.

Related Art

In an International Telecommunication Union Radio communication sector(ITU-R),

a standardization of International Mobile Telecommunication(IMT)-Advanced being a next mobile communication system after a thirdgeneration has been performed. The IMT-Advanced is aimed at supportingan Internet Protocol (IP) based multi-media service with a datatransmission rate of 1 Gbps in a stop and low speed moving state and adata transmission rate of 1 Gbps in a high speed moving state.

A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced(LTE-A) being an improved one of Long Term Evolution (LTE) based on anOFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (SingleCarrier-Frequency Division Multiple Access) transmission scheme as asystem standard satisfying requirements of IMT-Advanced. The LTE-A isone important candidate for IMT-Advanced.

In recent years, there is growing interest in a Device-to-Device (D2D)technology performing direct communication between devices. Inparticular, the D2D is attracting attention as a communicationtechnology for a public safety network. A commercial communicationnetwork has been rapidly changed to the LTE but a current public safetynetwork is based on a 2G technology in a collision problem and a costside with an existing communication standard. Request for the technologyclearance and an improved service induces an effort to improve thepublic safety network.

The public safety network has high service requirements (reliability andsecurity) as compared with a commercial communication network. Inparticular, when coverage of cellular communication is insufficient oris not used, there is a need for direct signal transmission/receptionbetween devices, that is, a D2D operation.

The D2D operation may be signal transmission/reception between adjacentdevices to have various advantages. For example, a D2D terminal mayperform data communication with a high transmission rate and low delay.Further, the D2D operation may distribute traffic converged in a basestation. If the D2D terminal serves as a relay, the D2D terminal mayserve to extend coverage of a base station.

Meanwhile, a network may broadcast information relating to adevice-to-device (D2D) operation in a specific cell, for example,information to indicate resources for receiving a D2D signal. It isneeded to specify how to operate when a terminal receives suchinformation.

SUMMARY OF THE INVENTION

Technical subject to be solved in the present invention provides a D2Doperation method performed by a user equipment (UE) in a wirelesscommunication system and the UE using the same.

In an aspect, a method for performance of a device-to-device (D2D)operation by a user equipment (UE) in a wireless communication system isprovided. The method comprises starting to use an exceptional resource,determining whether a condition for suspending the use of theexceptional resource is satisfied and when the condition for suspendingthe use is satisfied, suspending the use of the exceptional resource,and wherein the condition for suspending the use is a suspensioncondition in an RRC idle state.

The condition for suspending the use may be based on a timer.

The timer may include a first timer and a second timer, and wherein whenthe first timer is expired, starting to use of the exceptional resource,and when the first timer is expired, starting the second timer, andwherein the condition for suspending the use is when the second timer isexpired.

When the first timer is expired, and the second timer is in operation,the second timer may be restarted.

A value of the second timer may be signaled from a network.

A value of the second timer may be a predetermined value.

When the UE receives a service request from an upper layer, the secondtimer may be restarted.

The second timer may be different from the first timer.

The first timer may be a T300 timer.

The condition for suspending the use may be based on informationindicating suspension for the use of the exceptional resource, andwherein the information indicating suspension for the use of theexceptional resource is received from an upper layer.

The information indicating suspension for the use of the exceptionalresource may be information indicating that the UE uses the exceptionalresource.

The information indicating suspension for the use of the exceptionalresource may be information indicating that there are no resources to betransmitted for a direct communication with the UE.

The UE may receive, from the upper layer, information indicating thatthe exceptional resource is to be used, and wherein when the informationindicating that the exceptional resource is to be used is received, theuse of an exceptional resource is started.

When the timer is expired, the use of an exceptional resource may bestarted.

In another aspect, a user equipment (UE) is provided. The UE comprises aradio frequency (RF) unit configured to transmit and receive a radiosignal and a processor operatively coupled to the RF unit, and whereinthe processor is configured to: start to use an exceptional resource,determine whether a condition for suspending the use of the exceptionalresource is satisfied, and when the condition for suspending the use issatisfied, suspend the use of the exceptional resource, and wherein thecondition for suspending the use is a suspension condition in an RRCidle state.

According to this invention, the D2D operation performed by the UE inthe wireless system may be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 is a flowchart illustrating a process of rejecting an RRCconnection by a network.

FIG. 14 is a flowchart illustrating a process at the expiration of atimer during an RRC connection request.

FIG. 15 is a flowchart showing an example in which exceptional resourcesare continuously used.

FIG. 16 is a flowchart of an exceptional resource use restrictionprocess according to an embodiment of the present invention.

FIG. 17 is a flowchart of an exceptional resource use restrictionprocess based on a timer according to another embodiment of the presentinvention.

FIG. 18 is a flowchart illustrating an exceptional resource userestriction process based on the indication for suspending the use ofthe exceptional resource according to yet another embodiment of thepresent invention.

FIG. 19 is a flowchart illustrating an exceptional resource userestriction process based on the indication for suspending the use ofthe exceptional resource according to another embodiment of the presentinvention.

FIG. 20 is a block diagram showing a UE in which an embodiment of thepresent invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel The PHY layer isconnected to a medium access control (MAC) layer which is an upper layerof the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a procedure of defining thecharacteristics of a wireless protocol layer and channels in order toprovide specific service and configuring each detailed parameter andoperating method. An RB can be divided into two types of a Signaling RB(SRB) and a Data RB (DRB). The SRB is used as a passage through which anRRC message is transmitted on the control plane, and the DRB is used asa passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a procedure of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are mostessential and most frequently transmitted when other information isrequired to be obtained from a cell. UE first searches for an MIB afterdownlink synchronization. The MIB may include information, such as anSFN that supports downlink channel bandwidth, a PHICH configuration, andsynchronization and operates as a timing criterion and an eNB transmitantenna configuration. The MIB may be transmitted on a broadcast channel(BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. The remainingSIBs other than the SIB1 is included in a system information message andtransmitted. To map the SIBs to the system information message may beflexibly configured by a scheduling information list parameter includedin the SIB1. In this case, each of the SIBs is included in a singlesystem information message, and only SIBs having the same schedulingrequirement value (e.g. cycle) may be mapped to the same systeminformation message. Furthermore, a SystemInformationBlockType2 (SIB2)is always mapped to a system information message corresponding to thefirst entry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same cycle. The SIB1 and all the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may bededicated-signaled in the state in which it includes a parameterconfigured like an existing configured value. In this case, the SIB1 maybe included in an RRC connection reconfiguration message andtransmitted.

The SIB1 includes information related to UE cell access, and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers of a network, tracking area code (TAC) and a cellID, a cell barring status indicative of whether a cell is a cell onwhich camp-on is possible, the lowest reception level required within acell which is used as cell reselection criterion, and the transmissiontime and cycle of other SIBs.

The SIB2 may include radio resource configuration information common toall pieces of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detectinga change of system information to a primary cell (PCell) only. In asecondary cell (SCell), when a corresponding SCell is added, an E-UTRANmay provide all of pieces of system information related to an RRCconnection state operation through dedicated signaling. When systeminformation related to a configured SCell is changed, an E-UTRAN mayrelease an SCell that is taken into consideration and subsequently addthe changed system information. This may be performed along with asingle RRC connection reconfiguration message. An E-UTRAN may configureparameter values different from a value broadcasted within an SCell thathas been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation, and such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is an RRC idle state: The UE needs to be guaranteed so        that it has the valid versions of the MIB and the SIB1 in        addition to the SIB2 to SIB8. This may comply with the support        of a radio access technology (RAT) that is taken into        consideration.    -   If UE is an RRC connection state: The UE needs to be guaranteed        so that it has the valid versions of the MIB, the SIB1, and the        SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection procedure, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This procedure is called cell reselectiondifferently from the initial cell selection of the No. 2 procedure. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). Herein, the UE may be in RRC idle state. Moreover,when the UE transmits RRC connection request, the UE initiates timer,and the timer may be T300 in the 3GPP TS 36.331.

The network sends an RRC connection establishment message as a responseto the RRC connection request (S520). After receiving the RRC connectionestablishment message, the UE enters RRC connected mode. Herein, the UEmay stop the timer which is initiated in the step S510.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610).

As a response to the RRC connection reconfiguration message, the UEsends an RRC connection reconfiguration complete message used to checkthe successful completion of the RRC connection reconfiguration to thenetwork (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UEdoes not have preliminary information about a wireless channel.Accordingly, the UE searches for all wireless channels in order to findout a proper cell. The UE searches for the strongest cell in eachchannel Thereafter, if the UE has only to search for a suitable cellthat satisfies a cell selection criterion, the UE selects thecorresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection procedure. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a procedure, theUE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below.Following Equation 1 can be referred to as measurement for determiningwhether or not S-criterion is satisfied.Srxlev>0 AND Squal>0,where:Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P_(compensation),Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

In this case, in Equation 1, the variables may be defined as in Table 1below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX)−P_(PowerClass,) 0) (dB) P_(EMAX) Maximum TX power level anUE may use when transmitting on the uplink in the cell (dBm) defined asP_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE(dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are theresults of periodic discovery for a PLMN having higher priority while UEcamps on a normal cell within a VPLMN, and may be applied only when cellselection is evaluated. As described above, during the periodicdiscovery of a PLMN having higher priority, UE may perform cellselection evaluation using parameter values stored from another cell ofthe PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, theintensity or quality of a signal between the UE and a BS may be changeddue to the mobility of the UE or a change of a radio environment.Accordingly, if the quality of the selected cell is changed, the UE mayselect another cell providing better quality.

After the UE selects a specific cell through the cell selectionprocedure, the intensity or quality of a signal between the UE and a BSmay be changed due to a change in the mobility or wireless environmentof the UE. Accordingly, if the quality of the selected cell isdeteriorated, the UE may select another cell that provides betterquality. If a cell is reselected as described above, the UE selects acell that provides better signal quality than the currently selectedcell. Such a procedure is called cell reselection. In general, a basicobject of the cell reselection procedure is to select a cell thatprovides UE with the best quality from a viewpoint of the quality of aradio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency. For the intra-frequency cell reselection or theinter-frequency cell reselection, a network may provide UE with aNeighboring Cell List (NCL) used in cell reselection. The NCL includes acell-specific parameter (e.g., a cell-specific offset) used in cellreselection. For the intra-frequency or inter-frequency cellreselection, a network may provide UE with a cell reselection black listused in cell reselection.

The UE does not perform cell reselection on a cell included in the blacklist.

Ranking performed in a cell reselection evaluation procedure isdescribed below.

A ranking criterion used to give the priority of a cell is defined as inEquation 2.R _(s) =Q _(meas,s) +Q _(hyst) , R _(n) =Q _(meas,n) −Q _(offset)  [Equation 2]

In Equation 2, Rs is the ranking criterion of a serving cell on which UEnow camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s isthe quality value of the serving cell measured by the UE, Qmeas,n is thequality value of the neighboring cell measured by the UE, Qhyst is ahysteresis value for ranking, and Qoffset is an offset between the twocells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell res election.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this procedure, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selectionprocess (S801). The initial cell selection process may be performed whenthere is no stored cell information with respect to the PLMN or asuitable cell is not found.

If the suitable cell is not found in the initial cell selection process,the terminal transitions to an any cell selection state (S802). Theoptional cell selection state represents a state which does not camp onin both of a suitable cell and an acceptable cell. The optional cellselection state is a state attempted by the terminal in order to find anacceptable cell of an optional PLMN which may camp on. When the terminalfinds no cells which may camp on, the terminal is continuouslymaintained in an optional cell selection state until the acceptable cellis found.

If the suitable cell is found in the initial cell selection process, thestate transits to a normal camp state (S803). The normal camp staterepresents a state which camps on the normal cell. A paging channel isselected according to information given through system information tomotor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluationprocess (S804) is caused, the cell reselection evaluation process (S804)is performed. If a suitable cell is found in the cell reselectionevaluation process (S804), the terminal again transits to the normalcamp state (S803).

If an acceptable cell is found in the any cell selection state (S802),the terminal transits to an any cell camped state (S805). The any cellcamped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a pagingchannel according to information given through system information tomonitor, and may perform a cell reselection evaluation process (S806).If the acceptable cell is not found in the cell reselection evaluationprocess (S806), the terminal transits the any cell selection state(S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, aservice related to the D2D operation refers to Proximity based Services(ProSe). Hereinafter, the ProSe is an equivalent concept with the D2Doperation and the ProSe may be compatibly used with the D2D operation.The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe directdiscovery. The ProSe direct communication presents communicationperformed by two or more adjacent terminals. The terminals may performcommunication using a protocol of a user plane. A ProSe-enabled UE meansa UE for supporting a process related to requirements of the ProSe.Unless otherwise defined, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE represents aUE for supporting both of a public safety specified function and theProSe process. The non-public safety UE is a terminal which supports theProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UEdiscovers another ProSe-enabled UE. In this case, only ability of thetwo ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies aprocess where an EPC determines whether 2 ProSe enable terminals areclosed to each other, and reports the close state thereof the two ProSeenabled terminals.

Hereinafter, the ProSe direct communication may refer to D2Dcommunication, and the ProSe direct discovery may refer to D2Ddiscovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes aplurality of terminals having E-UTRAN, EPC, and ProSe applicationprogram, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may includean MME, an S-GW, a P-GW, a policy and charging rules function (PCRF),and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make anapplication function. The ProSe application server may communicate withan application program in the terminal. The application program in theterminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions butis not limited thereto.

-   -   Interworking via a reference point towards the 3rd party        applications    -   Authorization and configuration of the UE for discovery and        direct communication)    -   Enable the function of the EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of ProSe identities    -   Security related function    -   Provide control towards the EPC for policy related function    -   Provide function for charging (via or outside of EPC, e.g.,        offline charging))

Hereinafter, a reference point and a reference interface will bedescribed in a reference structure for the ProSe.

-   -   PC1: a reference point between a ProSe application program in        the terminal and a ProSe application program in a ProSe        application server. The PC1 is used to define signaling        requirements in an application level.    -   PC2: is a reference point between the ProSe application server        and a ProSe function. The PC2 is used to define an interaction        between the ProSe application server and a ProSe function. An        application data update of a ProSe database of the ProSe        function may be an example of the interaction.    -   PC3: is a reference point between the terminal and the ProSe        function. The PC3 is used to define an interaction between the        terminal and the ProSe function. Configuration for ProSe        discovery and communication may be an example of the        interaction.    -   PC4: is a reference point between an EPC and the ProSe function.        The PC4 is used to define an interaction between the EPC and the        ProSe function. The interaction lay illustrate when a path for        1:1 communication or a ProSe service for real time session        management or mobility management are authorized.    -   PC5: is a reference point to use control/user plane for        discovery, communication, and relay between terminals, and 1:1        communication.    -   PC6: is a reference point to use a function such as ProSe        discovery between users included in different PLMNs.    -   SGi: may be used for application data and application level        control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two publicsafety terminals may perform direct communication through a PC5interface. The communication mode may be supported in both of a case ofreceiving a service in coverage of E-UTRAN or a case of separating thecoverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cellcoverage. Referring to FIG. 10(b), the UE A may be located in the cellcoverage and the UE B may be located outside of the cell coverage.Referring to FIG. 10(c), both of UEs A and B may be located in the cellcoverage. Referring to FIG. 10(d), the UE A may be located in coverageof a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performedbetween terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packetin a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of apacket in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment(SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLClayer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MACheader may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect toresource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSedirect communication from a base station. The terminal should be in aRRC_CONNECTED state according to the mode 1 in order to transmit data.The terminal requests a transmission resource to the base station, andthe base station schedules a resource for scheduling assignment and datatransmission. The terminal may transmit a scheduling request to the basestation and may transmit a Buffer Status Report (ProSe BSR). The basestation has data which the terminal will perform the ProSe directcommunication and determines whether a resource for transmitting thedata is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminaldirectly selects a resource for the ProSe direct communication from aresource pool. The resource pool may be configured by a network or maybe previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when theterminal is in an RRC_CONNECTED state with the base station or islocated in a specific cell in an RRC_IDLE state, the terminal isregarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 isapplicable. If the terminal is located in the coverage, the mode 1 orthe mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station isconfigured, the terminal may change a mode from the mode 1 to the mode 2or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover whenthe ProSe enabled terminal discovers other neighboring ProSe enabledterminal and refers to D2D direction discovery or D2D discovery. In thiscase, an E-UTRA wireless signal through the PC 4 interface may be used.Hereinafter, information used for the ProSe direct discovery refers todiscovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer being an upper layer. Permission forannouncement and monitoring of discovery information is handled in theupper layer ProSe Protocol. Contents of discovery information aretransparent to an access stratum (AS). The ProSe Protocol allows onlyvalid discovery information to be transferred to the AS forannouncement.

An MAC layer receives discovery information from the upper layer ProSeProtocol. An IP layer is not used for transmitting the discoveryinformation. The MAC layer determines a resource used in order toannounce the discovery information received from the upper layer. TheMAC layer makes and sends a protocol data unit (MAC PDU) to a physicallayer. An MAC header is not added.

There are two types of resource assignments for announcing the discoveryinformation.

1. Type 1

The type 1 is a method assigned so that resources for announcing thediscovery information are not terminal-specific and the base stationprovides resource pool configuration for announcing the discoveryinformation to the terminals. The configuration may be included in asystem information block (SIB) to be signaled in a broadcast scheme.Alternatively, the configuration may be included in a terminal specificRRC message to be provided. Alternatively, the configuration may bebroadcast-signaled or terminal-specific signaled of a different layerfrom the RRC message.

The terminal selects a resource from an indicated resource pool toannounce discovery information using the selected resource. The terminalmay announce discovery information through a resource optionallyselected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discoveryinformation are terminal-specifically assigned. A terminal in aRRC_CONNECTED state may request a resource for announcing a discoverysignal to the base station through a RRC signal. The base station mayassign a resource for announcing a discovery signal as an RRC signal. Aresource for monitoring the discovery signal in a configured resourcepool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station mayreport a type 1 resource pool for announcing the discovery signal as anSIB. Terminals where ProSe direct discovery is allowed use a type 1resource pool for announcing the discovery information in the RRC_IDLEstate. Alternatively, the base station 2) reports that the base stationsupports the ProSe direct discovery through the SIB but may not providethe resource for announcing the discovery information. In this case, theterminal should enter the RRC_CONNECTED state for announcing thediscovery information.

With respect to a terminal in an RRC_CONNECTED state, the base stationmay configure whether to use a type 1 resource pool or a type 2 resourcepool for announcing the discovery information through a RRC signal.

Hereinafter, an RRC connection failure and a timer will be described.

FIG. 13 is a flowchart illustrating a process of rejecting an RRCconnection by a network.

Referring to FIG. 13, the UE may send an RRC Connection Request messageto the network (S1310). When the UE sends an RRC connection requestmessage to the network, it may start a timer, and the timer in this casemay be T300 of 3GPP TS36.331.

The network may send an RRC Connection Reject message in response to theRRC connection request (S1320). In this case, if the UE receives the RRCconnection reject message, the UE may suspend the timer started in stepS1310.

FIG. 14 is a flowchart illustrating a process at the expiration of atimer during an RRC connection request.

Referring to FIG. 14, the UE may send an RRC Connection Request messageto the network (S1410), and the UE in this case may mean a UE AS (AccessStratum).

When sending the RRC connection request message to the network, the UEmay start a timer (S1420), and the timer in this case may be T300 of3GPP TS 36.331.

If the timer expires when the UE sends an RRC connection request messageto the network, the UE informs an upper layer (Non-Access Stratum; NAS)that the RRC connection has failed (S1430). In addition, if the timerexpires, the UE may initialize an MAC, release the MAC configuration,and re-establish the RLC for all configured resource blocks. Inaddition, when the timer expires, the UE may store connectionestablishment failure information.

Hereinafter, the present invention will be described in detail.

The UE may use exceptional resources in specific cases, to ensurecontinuous service of ProSe operation. In this case, the exceptionalresource means resources available for an exceptional case defined inthe RRC_IDLE mode. The exceptional case will be described later. In theexceptional case, the UE may use the exceptional resource. Although theuse of exceptional resources is allowed to ensure a continuous servicelevel of ProSe direct communication, the use of exceptional resourcesshould be strictly restricted to protect cellular traffic frominterference that may be caused by ProSe direct communications usingexceptional resource.

If the RRC connection establishment attempt continues to fail,unexpected results may occur where the UE continuously uses the aboveexceptional resource as a cell broadcast transmission resource. It isnot intended or encouraged that the UE continuously uses an exceptionalresource (that is, the above-mentioned exceptional resource iscontinuously used as a cell broadcast transmission resource), and forthe cases that the UE continuously uses the exceptional resource, it isagainst the original purpose of the exceptional resource that wassupposed to use the exceptional resource only for ‘the exceptionalcase.’ However, when the network coverage is unstable (e.g., a largescale disaster), a situation may occur in which the UE continuously usesexceptional resources. Hereinafter, an example in which exceptionalresources are continuously used will be described in detail withreference to FIG. 15.

FIG. 15 is a flowchart showing an example in which exceptional resourcesare continuously used.

Referring to FIG. 15, in the RRC_IDLE state, the UE may receive aservice request for D2D from an upper layer (S1510). Herein, the UE maydenote a UE AS, the upper layer may denote a UE NAS, where the UE NASdenotes an upper layer in a UE, and the UE AS may be a lower layer inthe UE. Hereinafter, for convenience of description, it is assumed thatan upper layer (e.g., UE NAS) of a UE is referred to as ‘an upper layer’and a lower layer (e.g., a UE AS) of the UE is referred to as a ‘UE.’

When the UE receives a service request for D2D from an upper layer, theUE may transmit an RRC connection request message to the network for anRRC connection attempt (S1520), and performs an RRC connectionprocedure. In this case, the details of the RRC connection requestmessage and the RRC connection procedure are as described above.

When the UE transmits an RRC connection request message to the network,the UE starts a timer (S1530). The timer may mean T300 in TS 36.331, andthe T300 may be defined as in Table 2 below.

TABLE 2 Timer Start Stop At expiry T300 Transmission of Reception ofPerform the actions as specified RRCConnectionRequest RRCConnectionSetupor in 3GPP TS 36.331, 5.3.3.6 RRCConnectionReject message, and/or FIG.14 of present cell re-selection and upon application abortion ofconnection establishment by upper layers

If the UE fails the RRC connection attempt (i.e., the timer expires)until the timer started by the UE expires, the UE starts to use anexceptional resource for D2D (S1540).

If the timer expires, the UE informs the upper layer of RRC connectionestablishment failure information (S1550). The RRC connectionestablishment failure information is information indicating that the UEhas failed to make an RRC connection with the network, and the detailsof the RRC connection establishment failure information are as describedabove.

When the upper layer of the UE receives the RRC connection establishmentfailure information, the upper layer of the UE aborts the servicerequest (S1560).

Thereafter, when there is still data to be transmitted by the UE, theupper layer of the UE triggers the service request (S1570). That is, ifthere is still data to be transmitted by the UE, the UE receives aservice request for the D2D from the upper layer of the UE.

Thereafter, the UE repeats the above-mentioned steps S1520 to S1570until the RRC connection is established. In this case, the repetition ofsteps S1520 to S1570, that is, each service request process may refer toa new service request process.

As shown in FIG. 15, when an upper layer of the UE starts a new servicerequest process and the UE fails to make an RRC connection correspondingto the service request process until the timer expires, the UE mayconsider that reusing the exceptional resource which was started beingused from the step S1540. That is, if the problem (i.e., connectionestablishment failed until the timer expires) continues while the UEcontinues the connection establishment, triggering the service requestat the upper layer of the UE makes the UE continue to use theexceptional resource. As the UE continues to use the exceptionalresources, potential interference may occur in the network, and problemsmay be occurred in that potential interference may cause an increase incellular traffic.

Therefore, the situation in which the UE continuously uses anexceptional resource needs to be prevented, and restriction on the useof an exceptional resource by the UE is required. Hereinafter, a methodand apparatus for preventing the UE from continuously using exceptionalresources will be described in more detail.

FIG. 16 is a flowchart of an exceptional resource use restrictionprocess according to an embodiment of the present invention.

Referring to FIG. 16, the UE starts to use an exceptional resource(S1610). An entry condition of an exceptional case where the UE isallowed to use an exceptional resource for direct communication is asfollows.

-   -   Entry conditions in RRC_CONNECTED state: 1) Detect problems in        physical layer (3GPP TS 36.331, start of T310), 2) Radio Link        Failure (RLF) and RRC connection re-establishment start (3GPP TS        36.331, start of T311), 3) RRC connection re-establishment        request transmission (3GPP TS 36.331, start of T301). In this        case, details of T301, T310, and T311 are shown in Table 3        below.

TABLE 3 Timer Start Stop At expiry T301 Transmission of Reception of Goto RRC_IDLE RRCConnectionReestabilshment RRCConnectionReestablishmentRequest or RRCConnectionReestablishment Reject message as well as whenthe selected cell becomes unsuitable T310 Upon detecting physical Uponreceiving N311 If security is not activated: go to layer problems forthe consecutive in-sync indications RRC_IDLE else: initiate the PCelli.e. upon receiving from lower layers for the PCell, connectionre-establishment N310 consecutive out-of- upon triggering the handoverprocedure sync indications from procedure and upon initiating lowerlayers the connection re-establishment procedure T311 Upon initiatingthe RRC Selection of a suitable E-UTRA Enter RRC_IDLE connectionreestablishment cell or a cell using another RAT. procedure

-   -   Entry condition in the RRC_IDLE state: If SIB 18 does not        include the normal mode 2 resources and the UE performs the        connection procedure, the UE may use the exceptional resources        in the SIB 18 from the first T300 timer.

That is, in the RRC_IDLE state, the UE may start to use of anexceptional resource when the first timer expires, the first timer maybe T300 of TS 36.331 described above, and the UE may mean the UE AS.

Thereafter, the UE determines whether a condition for suspending use ofthe exceptional resource (S1620) is satisfied. The above-mentioned forsuspending the use of the exceptional resource, that is, an exitcondition of exceptional case is as follows.

-   -   Exit condition at RRC_CONNECTED: none of the T310, T311 and T301        timers are in operation.    -   Exit conditions in RRC_IDLE: 1) when the second timer expires        and 2) the UE receives indication for suspending the use of the        exceptional resource from the upper layer, and the second timer        may be intended to be a separate timer different from the first        timer, and the upper layer may mean a UE NAS. Details of the        condition for suspending the use of the exceptional resource        will be described later.

If the condition for suspending the use of the exceptional resource issatisfied, the UE suspends use of the exceptional resource used by theUE (S1630).

Hereinafter, the condition for suspending the use of the exceptionalresource 1) when the second timer expires, and 2) for the case where theUE receives the indication for suspending the use of the exceptionalresource from the upper layer, respectively, will be described.

FIG. 17 is a flowchart of an exceptional resource use restrictionprocess based on a timer according to another embodiment of the presentinvention.

Referring to FIG. 17, in the RRC_IDLE state, the UE may receive aservice request for D2D from an upper layer (S1710). In this case, theUE may denote a UE AS, and the upper layer may mean a UE NAS.

If the UE receives a service request for D2D from an upper layer, the UEmay transmit an RRC connection request message to the network for an RRCconnection attempt (S1715), and performs an RRC connection procedure. Inthis case, the details of the RRC connection request message and the RRCconnection procedure are as described above.

When the UE transmits an RRC connection request message to the network,the UE starts a first timer (S1720). The first timer may mean T300 in TS36.331.

If the UE fails the RRC connection attempt until the first timer startedby the UE expires and the UE is in the RRC_IDLE state, the UE starts touse the exceptional resource for D2D (S1725). The condition that the UEuses an exceptional resource for the D2D may be a case where the SIB 18does not include a normal mode 2 resource and the UE performs theconnection process. That is, if the first timer expires in the RRC_IDLEstate, the UE may start to use the exceptional resource. In this case,the UE may perform the D2D operation based on an automatic resourceselection in the exceptional resource pool.

When the first timer expires and the UE starts to use the exceptionalresource, the UE starts a second timer (S1730). More specifically, whenthe first timer expires, the UE may start the second timer if the secondtimer is not in operation. Alternatively, when the first timer expires,if the second timer is in operation, the UE may restart the secondtimer. In this case, the second timer is a timer for preventing the UEfrom continuously using the exceptional resource, and the second timermay be a separate timer different from the first timer.

More specifically, the second timer may be determined as follows.

-   -   The value of the second timer may be the value of the NAS timer        (e.g., NAS backoff timer)+alpha. In this case, the value of        alpha may be a predetermined value or a value signaled from a        serving cell.    -   The value of the second timer may be signaled from the serving        cell. As an example in which the value of the second timer is        signaled from the serving cell, the value of the second timer        may be included in the SIB.

If the second timer expires, the UE may suspend use of the exceptionalresource (S1735). In this case, in order to prevent the UE fromcontinuing to enter an exceptional situation and using the exceptionalresource again after the use of the exceptional resource is suspended,the UE may start a third timer which defers a time during which use ofthe exceptional resource is allowed after expiration of the secondtimer. That is, the UE may start the third timer after the second timerexpires, and the UE may be prohibited from using the exceptionalresource.

As an example of the above-described steps S1730 to S1735, the value ofthe second timer may be a predetermined value. That is, when the UE usesan exceptional pool in the RRC_IDLE state, the UE may use the exceptionpool only for a maximum of [xx] seconds. When the UE applies theexception pool to 3GPP TS36.331, it may be defined as the followingtable 4.

TABLE 4 5.X.4 Direct Communication transmission A UE capable of ProSeDirect Communication that is configured by upper layers to performDirect Communication transmission and has related data to be transmittedshall:  NOTE: Only if the UE is authorised, upper layers configure theUE to perform a particular ProSe activity.  1> If the cell used forProSe Direct Communication is suitable as defined in TS 36.304 [4]:   2>if T301, T310 or T311 is running and the PCell at which the UE detectedradio link failure broadcasts    SystemInformationBlockType18 includingcommTxPoolExceptional:    3> transmit the scheduling assignment and thecorresponding data, using the pool of resources     indicated bycommTxPoolExceptional, as specified in TS 36.321 [6];   2> else ifconfigured with commTxPoolNormalDedicated:    3> transmit the schedulingassignment and the corresponding data, using the pool of resources    indicated by commTxPoolNormalDedicated, as specified in TS 36.321[6];   2>    else if configured with sl-RNTI:    3> request E-UTRAN toassign transmission resources for direct communication, as specified inTS     36.321 [6];   2> else if the cell used for ProSe DirectCommunication concerns a non-serving cell broadcasting   SystemInformationBlockType18 including commTxPoolNormalCommon:    3>transmit the scheduling assignment and the corresponding data, using thepool of resources     indicated by commTxPoolNormalCommon, as specifiedin TS 36.321 [6];  eNote FFS whether to also support transmission on anon-serving frequency in idle mode also (might be     sufficient to justmove the two previous bullets below i.e. after the next bullet 2).   2>else if camped normally on the cell used for ProSe Direct Communication(RRC_IDLE):    3> if the cell used for ProSe Direct Communicationbroadcasts SystemInformationBlockType18     includingcommTxPoolNormalCommon:     4> transmit the scheduling assignment andthe corresponding data, using the pool of resources      indicated bycommTxPoolNormalCommon, as specified in TS 36.321 [6];    3> else if thelast connection establishment was initiated to request ProSe DirectCommunication     transmission resources and resulted in T300 expiry;and    3> if the cell on which the UE initiated connection establishmentbroadcasts      SystemInformationBlockType18 includingcommTxPoolExceptional:      4> if UE is currently not allowed to usecommTxPoolExceptionak and UE is not in prohibit       duration forcommTxPoolExceptional:       5> from the moment T300 expired untilreceiving an RCConnectionReconfiguration        includingProseCommConfig or until receiving an RRCConnectionRelease or an       RRCConnectionReject or until at maximum of [xx]s followed byprohibit duration [yy]        seconds during which UE is not allowed touse commTxPoolExceptional:        6> transmit the scheduling assignmentand the corresponding data, using the pool of         resourcesindicated by commTxPoolExceptional, as specified in TS 36.321 [6]; eNote  FFS whether there is a need for a (or another kind of) reject ofa Prose Direct Communication Tx     resource request, possiblyspecifically for the purpose of defining how long the UE may use the    exceptional resources during connection establishment.  eNote  TBCwhether exceptional resources are used generally (as agreed also for useduring     establishment when no mode is assigned yet), or used only ife.g. configured with mode 1.  1> else (out of coverage on ProSecarrier):   2> transmit the scheduling assignment and the correspondingdata, using the pool of resources that were    preconfigured, asspecified in TS 36.321 [6];

Referring to Table 4, in the RRC_IDLE mode, when the UE uses anexceptional resource for a maximum usable time (for example, 60 seconds)as described above, the UE is prohibited from using an exceptionalresource for [yy] seconds. Therefore, even if the first timer (e.g.,T300) is in operation or the T300 expires during the prohibition timeassociated with the second timer, the UE may not use an exceptionalresource. Thereafter, when the UE enters the RRC_CONNECTED mode, theabove prohibited time (i.e., [yy] seconds) is released. That is, thethird timer is stopped. In this case, the above-mentioned 60 secondsmeans the above [xx] seconds. That is, the UE may use the exceptionresource only in [xx] seconds described above, and after [xx] seconds,the UE does not use the exception resource for a maximum [yy] time evenif the T300 timer is in operation.

In step 1725, when the first timer expires and the UE starts to use theexceptional resource, the UE may inform the upper layer of RRCconnection establishment failure information (S1740).

After the UE informs the upper layer of the RRC connection establishmentfailure information, the UE may receive the service request from theupper layer (S1745). In this case, the UE may restart the second timer.Specific contents of the UE receiving the service request from the upperlayer are as described above.

When the UE receives a service request for D2D from an upper layer, theUE transmits an RRC connection request message to the network for an RRCconnection attempt (S1750), and may perform an RRC connection procedure.The details of the RRC connection procedure of the UE are as describedabove.

When the UE transmits an RRC connection request message to the network,the UE may start the first timer (S1755).

Thereafter, if the first timer started in step S1755 has expired, the UEmay inform the upper layer of RRC connection establishment failureinformation (S1760). In an exceptional resource use restriction processbased on the timer according to FIG. 17, each step (S1710 to S1760) maybe repeated.

FIG. 18 is a flowchart illustrating an exceptional resource userestriction process based on the indication for suspending the use ofthe exceptional resource according to yet another embodiment of thepresent invention.

Referring to FIG. 18, in the RRC_IDLE state, the UE receives a servicerequest for D2D from an upper layer (S1810). In this case, the UE maymean a UE AS, and the upper layer may mean a UE NAS.

When the UE receives a service request for D2D from an upper layer, theUE transmits an RRC connection request message to the network for an RRCconnection attempt (S1815), and performs an RRC connection procedure. Inthis case, the details of the RRC connection request message and the RRCconnection procedure are as described above.

When the UE transmits an RRC connection request message to the network,the UE starts the first timer (S1820). The first timer may mean T300 atTS 36.331.

If the UE fails the RRC connection attempt until the first timer startedby the UE expires and the UE is in the RRC_IDLE state, the UE starts touse the exception resource for D2D (S1825). The condition that the UEuses an exceptional resource for the D2D may be a case where the SIB 18does not include a normal mode 2 resource and the UE performs theconnection process. That is, if the first timer expires in the RRC_IDLEstate, the UE may start to use the exceptional resource. In this case,the UE may perform the D2D operation based on an automatic resourceselection in the exceptional resource pool.

When the UE starts to use an exceptional resource, the UE may indicateto the upper layer of the UE that the UE is using an exceptionalresource, and the UE may continue to use resources until the UE mayreceives from the upper layer, the indication for suspending the use ofthe exceptional resource

The UE may receive information for suspending the use of exceptionalresource from the upper layer (S1830). For example, the upper layer maysend exceptional resource use discontinuation information when theservice request to the UE for D2D fails five times consecutively.

In this case, the information for suspending the use of exceptionalresource is as follows.

-   -   information indicating that the higher layer instructs the UE to        suspend using the exceptional resource, or    -   information indicating that the upper layer has no resources to        transmit to the UE for direct communication, or    -   information indicating that the upper layer should suspend        transmission on direct communication, or    -   Information indicating that the upper layer should suspend        establishing an RRC connection for direct communication

When the UE receives the information for suspending the use ofexceptional resource from the upper layer, the UE suspends using theexceptional resource (S1835).

FIG. 19 is a flowchart illustrating an exceptional resource userestriction process based on the indication for suspending the use ofthe exceptional resource according to another embodiment of the presentinvention.

Referring to FIG. 19, in the RRC_IDLE state, the UE receives a servicerequest for D2D from an upper layer (S1910). In this case, the UE maymean a UE AS, and the upper layer may mean a UE NAS.

If the UE receives a service request for D2D from an upper layer, the UEtransmits an RRC connection request message to the network for an RRCconnection attempt (S1920), and performs an RRC connection procedure. Inthis case, the details of the RRC connection request message and the RRCconnection procedure are as described above.

The UE may receive information indicating the use of the exceptionalresource from an upper layer (e.g., UE NAS or UE Prose layer) (S1930).

Then, the UE starts to use the exceptional resource based on theinformation indicating the use of the exceptional resource (S1940).

When the UE starts using an exceptional resource, the UE may indicate tothe upper layer of the UE that the UE is using an exceptional resource,and the UE may continue to use the exceptional resource until the UEreceives an instruction from the upper layer to suspend the use of theexceptional resource.

The UE may receive the information for suspending the use of exceptionalresource from the upper layer (S1950). In this case, the information forsuspending the use of exceptional resource is as described above.

When the UE receives the information for suspending the use ofexceptional resource from the upper layer, the UE suspends using theexceptional resource (S1960).

FIG. 20 is a block diagram showing a UE in which an embodiment of thepresent invention is implemented.

Referring to FIG. 20, a UE 1100 includes a processor 1110, a memory1120, and a radio frequency unit (RF) unit 1130. Processor 1110implements the proposed functionality, process and/or method. Forexample, processor 1110 may start to use of the exceptional resourcethrough RF unit 1130.

In this case, the processor 1110 may determine whether a condition forsuspending the use of the exceptional resource is satisfied through theRF unit 1130.

In this case, if the condition for suspending the use of the exceptionalresource is satisfied, the processor 1110 may suspend the use of theexception resource through the RF unit 1130.

Further, in the RRC_IDLE state, the processor 1110 may receive a servicerequest for the D2D from the upper layer through the RF unit 1130.

In this case, the processor 1110 may transmit an RRC connection requestmessage for an RRC connection attempt through the RF unit 1130 to thenetwork.

In this case, the processor 1110 starts a first timer through the RFunit 1130.

In this case, the processor 1110 starts to use the exceptional resourcefor the D2D through the RF unit 1130 until the first timer expires.

In this case, when the processor 1110 starts to use an exceptionalresource through the RF unit 1130, it starts a second timer.

In this case, the processor 1110 may suspend the use of the exceptionalresource through the RF unit 1130 when the second timer expires.

The RF unit 1130 is coupled to the processor 1110 to transmit andreceive radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for performing device-to-device (D2D)operation in a wireless communication system, the method performed by auser equipment (UE) and comprising: starting a first timer; if the firsttimer is expired: starting or restarting a second timer, and performingthe D2D operation using an exceptional resource; determining whether acondition for stopping use of the exceptional resource is satisfiedbased on the whether the second timer is expired; and stopping the userof the exceptional resource when performing the D2D operation, based onthe determination, wherein the condition for stopping the use is a stopcondition in an RRC idle station, wherein, if the second timer isexpired, the UE determines that the condition for stopping the use ofthe exceptional resource is satisfied, and wherein, if the first timeris expired while the second timer is not running, the UE starts thesecond timer, and if the first timer is expired while the second timeris already running, the UE restarts the second timer.
 2. The method ofclaim 1, wherein a value of the second timer is signaled from a network.3. The method of claim 1, wherein a value of the second timer is apredetermined value.
 4. The method of claim 1, wherein when the UEreceives a service request from an upper layer, the second timer isrestarted.
 5. The method of claim 1, wherein the second timer isdifferent from the first timer.
 6. The method of claim 1, wherein thefirst timer is a T300 timer.
 7. The method of claim 1, wherein thecondition for stopping the use is based on information indicating stopfor the use of the exceptional resource, and wherein the informationindicating stop for the use of the exceptional resource is received froman upper layer.
 8. The method of claim 7, wherein the informationindicating stop for the use of the exceptional resource is informationindicating that the UE uses the exceptional resource.
 9. The method ofclaim 7, wherein the information indicating stop for the use of theexceptional resource is information indicating that there are noresources to be transmitted for a direct communication with the UE. 10.The method of claim 7, wherein the UE receives, from the upper layer,information indicating that the exceptional resource is to be used, andwherein when the information indicating that the exceptional resource isto be used is received, the use of an exceptional resource is started.11. A user equipment (UE), the UE comprising: a transceiver thattransmits and receives a radio signal; and a processor operativelycoupled to the transceiver, wherein the processor: starts a first timer;if the first timer is expired: starts or restarts a second timer, andperforms device-to-device (D2D) operation using an exceptional resource;determines whether a condition for stopping use of the exceptionalresource is satisfied based on whether the second timer is expired; andstops the use of the exceptional resource when performing the (D2D)operation, based on the determination, wherein the condition forstopping the use is a stop condition in an RRC idle state, wherein, ifthe second timer is expired, the UE determines that the condition forstopping the use of the exceptional resource is satisfied, and wherein,if the first timer is expired while the second timer is not running, theUE starts the second timer, and if the first timer is expired while thesecond timer is already running, the UE restart the second timer.